Project Summary Major depressive disorder (MDD) is a leading cause of disability worldwide, and it is characterized by vast heterogeneity. As approximately 35-50% of adults will experience recurrent episodes, recent research has sought to parse this heterogeneity by focusing on differences among individuals experiencing single episode (sMDD) versus recurrent (recMDD) MDD. Recurrent MDD is associated with increased rates of comorbidity, greater psychosocial dysfunction, and worse treatment response; consequently, identifying mechanisms implicated in recMDD may provide novel clinical targets for early identification and treatment. The National Institute of Mental Health's Research Domain Criteria (RDoC) provides an innovative framework to investigate promising mechanisms that may underlie recMDD. Constructs within the RDoC domains of the Positive Valence System (PVS, e.g., blunted initial reward responsiveness) may be particularly important for the maintenance and recurrence of MDD. However, the majority of this research has relied on monetary as opposed to social reward (e.g., social acceptance) paradigms despite seminal research directly linking social processes to recMDD. The aims of the present study therefore focus on RDoC constructs that intersect PVS and Social Processes (e.g., affiliation) domains. Specifically, the first aim of the study is to utilize an ecologically valid peer feedback task (i.e., peer acceptance vs. rejection) to elicit event-related potentials and associated neural processes that are believed to confer risk for recurrent episodes of depression. For the second aim, an innovative NIH funded, ecological momentary assessment smartphone app (Beiwe) that measures both `active' (e.g., explicit probes of in vivo affect and stress) and `passive' (e.g., frequency of incoming/outgoing text messages and phone calls) data will be used to test whether a blunted response to social reward among recMDD adults predicts psychosocial dysfunction. For the third aim, given that interpersonal stressors are potent predictors of depression recurrence, we will test whether blunted response to social reward in recMDD individuals prospectively predicts interpersonal stressors over a 6-month follow-up period. Ultimately, testing whether social reward processing deficits prospectively predict interpersonal stress and depressive symptoms may elucidate the pathway to depression recurrence. One limitation of past research on mechanisms of recMDD is that it has largely focused on currently symptomatic individuals. These studies therefore cannot ascertain whether deficits are a function of current symptomatology or whether they also would persist into remission. Thus, the proposed study will test the three aims in remitted recMDD individuals, remitted sMDD individuals, and healthy adults. As a whole, the proposed project has the potential to improve our understanding of neurophysiological markers and social processes that contribute to depression recurrence, which may lead to key developments for early identification and preventative intervention approaches for individuals at risk for depression recurrence.